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00:00:10,934 --> 00:00:13,050
We're also going to talk a lot about light
in this class.

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00:00:13,050 --> 00:00:15,930
And so, you need to be familiar with the
various

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00:00:15,930 --> 00:00:19,450
ways of describing the different kinds of
light that are found.

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00:00:20,560 --> 00:00:25,270
A light ray consists of waves that

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00:00:25,270 --> 00:00:28,200
we'll talk more about in future
lectures.

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00:00:28,200 --> 00:00:31,100
But one way to characterize the different
kinds of

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00:00:31,100 --> 00:00:36,160
light waves is by the wavelength,

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00:00:36,160 --> 00:00:39,030
the distance between two adjacent peaks.

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00:00:39,030 --> 00:00:43,330
It's given the Greek letter lambda to
describe the wavelength.

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00:00:43,330 --> 00:00:49,230
And the units of wavelength are the number
of cycles per centimeter.

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00:00:49,230 --> 00:00:50,360
I have this upside down here.

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00:00:52,240 --> 00:00:57,080
No, centimeter, centimeters per cycle,
that's centimeters per cycle.

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00:00:57,080 --> 00:01:01,360
And a lot of times, the units of cycle are
left

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00:01:01,360 --> 00:01:02,580
out, sort of implicit.

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00:01:02,580 --> 00:01:04,710
And so sometimes very often you will

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00:01:04,710 --> 00:01:08,160
see wavelength described in units of
centimeters.

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00:01:11,310 --> 00:01:15,110
Another way to describe light is in terms
of the frequency of light.

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00:01:15,110 --> 00:01:19,250
So light is moving at some speed C, the
speed of light.

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00:01:19,250 --> 00:01:21,530
And so, if you're just sitting here
watching this light

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00:01:21,530 --> 00:01:24,410
go past you, you can count the peaks as
they're coming.

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00:01:24,410 --> 00:01:26,920
And so, the frequency of the light, which
is given

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00:01:26,920 --> 00:01:32,110
the Greek letter nu, has units of cycles
per second.

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00:01:32,110 --> 00:01:36,780
Or, since they tend to drop the cycles,
you can see this written as just

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00:01:36,780 --> 00:01:40,685
one over seconds, or seconds to the minus
1.

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00:01:40,685 --> 00:01:42,510
Can be confusing if you don't know what's
going on.

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00:01:42,510 --> 00:01:43,480
But it's not that hard.

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00:01:45,960 --> 00:01:49,130
So if we want to get some more factor
label

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00:01:49,130 --> 00:01:52,350
practice at converting units, this is a
good example.

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00:01:52,350 --> 00:01:58,100
Let's say we have a wavelength of light
and we want to calculate its frequency.

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00:01:58,100 --> 00:02:01,030
So the frequency has units of cycles per
second.

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00:02:01,030 --> 00:02:02,630
So that's what we want.

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00:02:02,630 --> 00:02:07,670
And looking at the wavelength, 

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00:02:07,670 --> 00:02:11,020
the wavelength itself is centimeters per
cycle,

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00:02:11,020 --> 00:02:13,850
but it looks like we want the cycles to be
on top.

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00:02:13,850 --> 00:02:18,870
So I'm going to put the wavelength
upside-down, one over lambda.

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00:02:18,870 --> 00:02:20,630
And so that gives us units here of

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00:02:20,630 --> 00:02:24,850
cycles per centimeter, the inverse of 
lambda there.

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00:02:24,850 --> 00:02:31,940
And then the speed of light C can be used
to cancel the centimeters and get us 

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00:02:31,940 --> 00:02:37,570
the cycles per second that we're looking for.

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00:02:37,570 --> 00:02:40,490
Another metric
of light that turns out to be useful


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00:02:40,490 --> 00:02:44,510
when we start talking about infrared light is
called the wave number.

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00:02:44,510 --> 00:02:46,560
And the wave number is simply the amount,

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00:02:46,560 --> 00:02:51,540
the number of waves within 1 centimeter of
distance.

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00:02:51,540 --> 00:02:53,860
So you count the number of peaks in there,

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00:02:53,860 --> 00:02:57,390
and the units of that are cycles per
centimeter.

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00:02:57,390 --> 00:03:02,660
Or by neglecting, ignoring the cycles
there, it's oftentimes written as 

48
00:03:02,660 --> 00:03:05,940
1 / cm, or cm raised to the
power of minus 1.

49
00:03:05,940 --> 00:03:10,520
The wave number is just given the
regular letter n.